XXXIV INTRODUCTION. 



clear atmospbere common to more northern latitudes.' The mean 

 temperature of winter does not fall below freezing point. 



Plants which would not bear complete exposure to frost will often 

 survive, with slight shelter, frosts of short duration ; and near the 

 western coasts, where the influence of the sea has greater effect, 

 especially in mitigating the winter, comparatively tender plants 

 flourish in the open air throughout the year. A warm winter 

 is an essential condition for the existence of tender plants with 

 perennial stems. In the neighbourhood of London, on the other 

 hand, the semi-spontaneous exotic plants which belong to the vege- 

 tation of climates with a higher mean temperature are necessarily 

 annuals. Many of these are more abundant in some years than 

 in others, a warm spring being essential to allow them to reach 

 maturity before the first frosts.* Provided that the summer heat is 

 sufficient to allow them to ripen their seeds, annuals are capable of 

 a more extended northern duration than perennials. With regard to 

 perennials the following remarks may be quoted from Mr. Baker : — 

 *In general terms, the polar limit of species liable to be killed by 

 frost, runs across Europe, from N.W. to S.E., diagonally with the 

 parallels of latitude; and to sum up, in a single comprehensive 

 phrase, the relations of the British to the Continental flora, we may 

 say, that the north limits of the plants, from the nature of the case, 

 as regulated by temperature, radiate from our island, like the spokes 

 of a wheel from the axis.' (Flora of NoHhumherland and Durhamy 

 p. 57.) 



2. Raiffall. — Stations on the west coasts seldom have less than 

 40 inches rain, but, passing from west to east, the fall decreases gra- 

 dually if the country is level. At about three-fourths of the distance 

 across the country, the minimum of 20 is reached, and then the fall 

 increases again until we arrive at the east coast, where the fall is 

 25 inches. (Symons, Rain^ p. 55.) Thus Barnstaple, Chapel-en-le- 

 Frith, Plymouth, Shetland, Cork, Waterford, have an approximate 

 mean annual rainfall of 40 inches. Ackworth, Epping, Hereford, 

 Horncastle, Oxford, Sunderland, Elgin, 25 inches. Bushey, Cobham, 

 London, Norwich, Thirsk, Hawarden, Edinburgh, 24 inches. Lincoln, 

 Southwell, Stamford, 20 inches (p. 52). 



The mean annual rainfall at Chiswick, deduced from observations 

 made from 1826 to 1842, was 24-16 inches (Daniell's Meteorology ^ 

 vol. ii. p. 316). The greatest rainfall during that time was 30-97 in 

 1841. The least 18-87 in 1840. At Tottenham, from Howard's 



* See ' Exotic Plants about London in 1865 ' {Seem. Journ. of Bot. iv. 147). 



